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arXiv:quant-ph/0101009 · Journal of Physics A Mathematical and General

Quantum networks for concentrating entanglement

Phillip Kaye, Michele Mosca

Published Aug 24, 2001• 21 citations
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Abstract

If two parties, Alice and Bob, share some number, n , of partially entangled pairs of qubits, then it is possible for them to concentrate these pairs into some smaller number of maximally entangled states. We present a simplified version of the algorithm for such entanglement concentration , and we describe efficient networks for implementing these operations.

DOI
10.1088/0305-4470/34/35/319

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If two parties, Alice and Bob, share some number, n , of partially entangled pairs of qubits, then it is possible for them to concentrate these pairs into some smaller number of maximally entangled states. We present a simplified version of the algorithm for such entanglement concentration , and we describe efficient networks for implementing these operations.
In the practical context of quantum networks, the most reliable method of transmitting quantum information is via teleportation because quantum states are highly sensitive. However, teleportation consumes a shared maximally entangled state. Two parties Alice and Bob located at separate nodes that wish to reestablish their shared entanglement will not send entangled qubits directly to achieve this goal, but rather employ a more efficient mechanism that ensures minimal time resources. In this paper, we present a quantum routing scheme that exploits entanglement swapping to reestablish consumed entanglement. It improves and generalizes previous work on the subject and reduces the entanglement distribution time by a factor of 4 k in an arbitrary scale quantum network, where N = 4 k - 1 is a required number of quantum nodes located between source and destination. In addition, k is the greatest positive integer considered by Alice or Bob, such that afterwards they choose N quantum switches.
If two separated observers are supplied with entanglement, in the form of n pairs of particles in identical partly entangled pure states, one member of each pair being given to each observer, they can, by local actions of each observer, concentrate this entanglement into a smaller number of maximally entangled pairs of particles, for example, Einstein-Podolsky-Rosen singlets, similarly shared between the two observers. The concentration process asymptotically conserves entropy of entanglement---the von Neumann entropy of the partial density matrix seen by either observer---with the yield of singlets approaching, for large n, the base-2 entropy of entanglement of the initial partly entangled pure state. Conversely, any pure or mixed entangled state of two systems can be produced by two classically communicating separated observers, drawing on a supply of singlets as their sole source of entanglement. \textcopyright{} 1996 The American Physical Society.
An experimental scheme for concentrating entanglement in partially entangled photon pairs is proposed. In this scheme, two separated parties obtain one maximally entangled photon pair from two previously shared partially entangled photon pairs by local operations and classical communication. A practical realization of the proposed scheme is discussed, which uses imperfect photon detectors and spontaneous parametric down-conversion as a photon source. This scheme also works for purifying a class of mixed states.
We present a new scheme for investigating the usefulness of non-maximally entangled states for multi-party quantum state sharing in a simple and elegant manner. In our scheme, the sender, Alice shares n various probabilistic channels composed of non-maximally entangled states with n agents in a network. Our protocol involves only Bell-basis measurements, single qubit measurements, and a two-qubit unitary transformation operated by free optional agents. Our scheme is a more convenient realization because no other multipartite joint measurements are needed. Furthermore, in our scheme various probabilistic channels lessen the requirement for quantum channels, which makes it more practical for physical implementation.